Extender Manual

Introduction
This document explains how to extend Boost.Build to accomodate your local requirements. Let's start with quite simple, but realistic example.
Say you're writing an application which generates C++ code. If you ever did this, you know that it's not nice. Embedding large portions of C++ code in string literals is very awkward. A much better solution is:
Write the template of the code to be generated, leaving placeholders at the points which will change
Access the template in your application and replace placeholders with appropriate text.
Write the result.
It's quite easy to achieve. You write special verbatim files, which are just C++, except that the very first line of the file gives a name of variable that should be generated. A simple tool is created which takes verbatim file and creates a cpp file with a single char* variable, which name is taken from the first line of verbatim file, and which value is properly quoted content of the verbatim file.
Let's see what Boost.Build can do.
First off, Boost.Build has no idea about "verbatim files". So, you must register a new type. The following code does it:

import type ;
type.register VERBATIM : verbatim ;

The first parameter to 'type.register' gives the name of declared type. By convention, it's uppercase. The second parameter is suffix for this type. So, if Boost.Build sees "code.verbatim" in the list of sources, it knows that it's of type VERBATIM.
Lastly, you need a tool to convert verbatim files to C++. Say you've sketched such a tool in Python. Then, you have to inform Boost.Build about the tool. The Boost.Build concept which represents a tool is generator.
First, you say that generator 'inline-file' is able to convert VERBATIM type into C++:

import generators ;
generators.register-standard verbatim.inline-file : VERBATIM : CPP ;

Second, you must specify the commands to be run to actually perform convertion:

actions inline-file
{
    "./inline-file.py" $(<) $(>)
}

Now, we're ready to tie it all together. Put all the code above in file "verbatim.jam", add "import verbatim ;" to "project-root.jam", and it's possible to write the following in Jamfile:

exe codegen : codegen.cpp class_template.verbatim usage.verbatim ;

The verbatim files will be automatically converted into C++ and linked it.
In the subsequent sections, we will extend this example, and review all the mechanisms in detail. The complete code is available in example/customization directory.

Target types
The first thing we did in the intruduction was declaring a new target type:

import type ;
type.register VERBATIM : verbatim ;

The type is the most important property of a target. Boost.Build can automatically generate necessary build actions only because you specify the desired type (using the different main target rules), and because Boost.Build can guess the type of sources from their extensions.
The first two parameters for the type.register rule are the name of new type and the list of extensions associated with it. A file with an extension from the list will have the given target type. In the case where a target of the declared type is generated from other sources, the first specified extension will be used. This behaviour can be changed using the type.set-generated-target-suffix rule.
Something about 'main' types.
Something about base types.

Scanners
Sometimes, a file can refer to other files via some include mechanism. To make Boost.Build track dependencies to the included files, you need to provide a scanner. The primary limitation is that only one scanner can be assigned to a target type.
First, we need to declare a new class for the scanner:

class verbatim-scanner : common-scanner
{
    rule pattern ( )
    {
        return "//###include[ ]*\"([^\"]*)\"" ;
    }
}

All the complex logic is in the common-scanner class, and you only need to override the method which returns the regular expression to be used for scanning. The paranthethis in the regular expression indicate which part of the string is the name of the included file.
After that, we need to register our scanner class:

scanner.register verbatim-scanner : include ;

The value of the second parameter, in this case include, specifies which properties contain the list of paths which should be searched for the included files.
Finally, we assign the new scaner to the VERBATIM target type:

type.set-scanner VERBATIM : verbatim-scanner ;

That's enough for scanning include dependencies.

Tools and generators
This section will describe how Boost.Build can be extended to support new tools.
For each additional tool, a Boost.Build object called generator must be created. That object has specific types of targets which it accepts an produces. Using that information, Boost.Build is able to automatically invoke the generator. For example, if you declare a generator which takes a target of the type D and produces a target of the type OBJ, when placing a file with extention .d in a list of sources will cause Boost.Build to invoke your generator, and then to link the resulting object file into an application. (Of course, this requires that you specify that the .d extension corresponds to the D type.)
Each generator should be an instance of a class derived from the generator class. In the simplest case, you don't need to create a derived class, but simply create an instance of the generator class. Let's review the example we've seen in the introduction.

import generators ;
generators.register-standard verbatim.inline-file : VERBATIM : CPP ;
actions inline-file
{
    "./inline-file.py" $(<) $(>)
}

We declare a standard generator, specifying its id, the source type and the target type. When invoked, the generator will create a target of type CPP which will have the source target of type VERBATIM as the only source. But what command will be used to actually generate the file? In bjam, actions are specified using named "actions" blocks and the name of the action block should be specified when creating targets. By convention, generators use the same name of the action block as their own id. So, in above example, the "inline-file" actions block will be use to convert the source into the target.
There are two primary kinds of generators: standard and composing, which are registered with the generators.register-standard and the generators.register-composing rules, respectively. For example:

generators.register-standard verbatim.inline-file : VERBATIM : CPP ;
generators.register-composing mex.mex : CPP LIB : MEX ;

The first generators takes a single source of type VERBATIM and produces a result. The second generator takes any number of sources, which can have either the CPP or the LIB type. Composing generators are typically used for generating top-level target type. For example, the first generator invoked when building an exe target is a composing generator corresponding to the proper linker.
You should also know about two specific function for registering generators: generators.register-c-compiler and generators.register-linker. The first sets up header dependecy scanning for C files, and the seconds handles various complexities like searched libraries. For that reason, you should always use those functions when adding support for compilers and linkers.
(Need a note about UNIX)

Custom generator classes
The standard generators allows you to specify source and target types, action, and a set of flags. If you need anything more complex, you need to create a new generator class with your own logic. Then, you have to create an instance of that class and register it. Here's an example how you can create your own generator class:

class custom-generator : generator
{
    rule __init__ ( * : * )
    {
        generator.__init__ $(1) : $(2) : $(3) : $(4) : $(5) : $(6) : $(7) : $(8) : $(9) ;
    }
}

generators.register 
  [ new custom-generator verbatim.inline-file : VERBATIM : CPP ] ;

This generator will work exactly like the verbatim.inline-file generator we've defined above, but it's possible to customize the behaviour by overriding methods of the generator class.
There are two methods of interest. The run methods is responsible for overall process - it takes a number of source targets, converts them the the right types, and creates the result. The generated-targets method is called when all sources are converted to the right types to actually create the result.
The generated-target method can be overridden when you want to add additional properties to the generated targets or use additional sources. For example (which is real), you have a tool for analysing programs, which should be given a name of executable and the list of all sources. Naturally, you don't want to list all source files manually. Here's how the generated-target method can find the list of sources automatically:

class itrace-generator : generator {
....
    rule generated-targets ( sources + : property-set : project name ? )
    {
        local leafs ;
        local temp = [ virtual-target.traverse $(sources[1]) : : include-sources ] ;
        for local t in $(temp)
        {
            if ! [ $(t).action ]
            {
                leafs += $(t) ;
            }
        }
        return [ generator.generated-targets $(sources) $(leafs)
          : $(property-set) : $(project) $(name) ] ;
    }
}
generators.register [ new itrace-generator nm.itrace : EXE : ITRACE ] ;

The generated-targets rule will be called with a single source target of type EXE. The call to the virtual-target.traverse will return all targets the executable depends on, and we further find files which are not produced from anything. The found targets are added to the sources.
The run method can be overriden to completely customize the way generator works. In particular, the conversion of sources to the desired types can be completely customized. Here's another real example. Tests for the Boost Python library usually consist of two parts: a Python program and a C++ file. The C++ file is compiled to Python extension which is loaded by the Python program. But in the likely case that both files have the same name, the created Python extension must be renamed. Otherwise, Python program will import itself, not the extension. Here's how it can be done:

rule run ( project name ? : property-set : sources * : multiple ? )
{       
    local python ;
    for local s in $(sources)
    {
        if [ $(s).type ] = PY
        {
            python = $(s) ;
        }
    }
    
    local libs ;
    for local s in $(sources)
    {
        if [ type.is-derived [ $(s).type ] LIB ]
        {
            libs += $(s) ;
        }
    }
    
    local new-sources ;
    for local s in $(sources)
    {
        if [ type.is-derived [ $(s).type ] CPP ] 
        {
            local name = [ $(s).name ] ;
            if $(name) = [ $(python).name ] 
            {
                name = $(name)_ext ;
            }                                
            new-sources += [ generators.construct $(project) $(name) :
              PYTHON_EXTENSION : $(property-set) : $(s) $(libs) ] ;
        }
    }
        
    result = [ construct-result $(python) $(new-sources) : $(project) $(name) 
                 : $(property-set) ] ;        
}

First, we separate all source into python files, libraries and C++ sources. For each C++ source we create a separate Python extension by calling generators.construct and passing the C++ source and the libraries. At this point, we also change the extension's name, if necessary.

Features
Often, we need to control the options passed the invoked tools. This is done with features. Consider an example:

# Declare a new feature
import feature : feature ;
feature verbatim-options : : free ;

# Cause the value of the 'verbatim-options' feature to be
# available as 'OPTIONS' variable inside verbatim.inline-file
import toolset : flags ;
flags verbatim.inline-file OPTIONS <verbatim-options> ;

# Use the "OPTIONS" variable
actions inline-file
{
    "./inline-file.py" $(OPTIONS) $(<) $(>)
}

We first define a new feature. Then, the flags invocation says that whenever verbatin.inline-file action is run, the value of the verbatim-options feature will be added to the OPTIONS variable, an can be used inside the action body. You'd need to consult online help (--help) to find all the features of the toolset.flags rule.
Although you can define any set of features and interpret their values in any way, Boost.Build suggests the following coding standard for designing features.
Most features should have a fixed set of values, which is portable (tool neutral) across the class of tools they are designed to work with. The user does not have to adjust the values for a exact tool. For example, <optimization>speed has the same meaning for all C++ compilers and the user does not have to worry about the exact options which are passed to the compiler's command line.
Besides such portable features there are special 'raw' features which allow the user to pass any value to the command line parameters for a particular tool, if so desired. For example, the <cxxflags> feature allows to pass any command line options to a C++ compiler. The <include> feature allows to pass any value to the -I and the interpretation is tool-specific. (There an example of very smart usage of that feature). Of course one should always strive to use the portable features but these should still be provided as a backdoor just to make sure Boost.Build does not take away any control from the user.
Some of the reasons why portable features are better are:
Since a portable feature have a fixed set of value, you will be able to build your project with two different settings of the feature. Boost.Build will automatically use two different directories for produced files. If you pass raw compiler options, Boost.Build assumes you know what you are doing, and would not care about what options are passed.
Unlike "raw" features, you don't need to use specific compiler flags in Jamfile, and it will more likely work on other systems.

Steps for adding a feauture
Adding a feature requires three steps:
Declaring a feature. For that, the "feature.feature" rule is used. You should have to decide on the set of feature attributes:
if feature has several values, and significally affects build, make it "propagated", so that whole project is build with the same value by default
if a feature does not have a fixed list of values, it must be "free".
if feature is used to refer to a path, it must be "path".
if feature is used to refer to some target, it must be "dependency".
Converting the feature value into variable. To use feature in build action, it must be converted into a variable, accessible in build action. This is accomplished by "toolset.flags" rule.
Using the variable. The variable set in step 2 can be used in build action to form command parameters or files.

Another example
Here's an another example. Let's see how we can make a feature which refers to a target. For example, when linking dynamic libraries on windows, one sometimes needs to specify "DEF file", telling what functions should be exported. It would be nice to use this file like this:

        lib a : a.cpp : <def-file>a.def ;

Actually, this feature is already supported, but anyway...
Since the feature refers to a target, it must be "dependency".

feature def-file : : free dependency ;

One of the toolsets which cares about DEF files is msvc. The following line should be added to it.

flags msvc.link DEF_FILE <def-file> ;

Since the DEF_FILE variable is not used by the msvc.link action, we need to modify it to be:

actions link bind DEF_FILE
{
    $(.LD) .... /DEF:$(DEF_FILE) ....
}

Note the "bind DEF_FILE" part. It tells bjam that DEF_FILE refers to a file, otherwise the variable will contain internal target name, which is not likely to make sense for the linker.
We've almost done, but should stop for a small workaround. Add the following code to msvc.jam

rule link
{
    DEPENDS $(<) : [ on $(<) return $(DEF_FILE) ] ;
}

This is needed to accomodate some bug in bjam, which hopefully will be fixed one day.

Variants and composite features.
Sometimes you want to create a shorcut for some set of features. For example, release is a value of the variant and is a shortcut for a set of features.
.
It is possible to define your build variants. For example:

variant crazy : <optimization>speed <inlining>off
                <debug-symbols>on <profiling>on ;

will define a new variant with the specified set of properties. You can also extend an existing variant:

variant super_release : release : <define>USE_ASM ;

In this case, super_release will expand to all properties specified by release, and the additional one you've specified.
You are not restricted to using the variant feature only. Here's example which defines a brand new feature:

feature parallelism : mpi fake none : composite link-incompatible ;
feature.compose <parallelism>mpi : <library>/mpi//mpi/<parallelism>none ;
feature.compose <parallelism>fake : <library>/mpi//fake/<parallelism>none ;

This will allow you to specify value of feature parallelism, which will expand to link to the necessary library.

Main target rules
The main target rule is what creates a top-level target, for example "exe" or "lib". It's quite likely that you'll want to declare your own and there are as many as three ways to do that.
The first is the simplest, but is sufficient in a number of cases. Just write a wrapper rule, which will redirect to any of the existing rules. For example, you have only one library per directory and want all cpp files in the directory to be compiled. You can achieve this effect with:

lib codegen : [ glob *.cpp ] ;

but what if you want to make it even simple. Then, you add the following definition to the project-root.jam file:

rule glib ( name : extra-sources * : requirements * )
{
    lib $(name) : [ glob *.cpp ] $(extra-sources) : $(requirements) ;
}

which would allow to reduce Jamfile to

glib codegen ;

The second approach is suitable when your target rule should just produce a target of specific type. Then, when declaring a type you should tell Boost.Build that a main target rule should be created. For example, if you create a module "obfuscate.jam" containing:

import type ;
type.register OBFUSCATED_CPP  : ocpp : : main ;

import generators ;
generators.register-standard obfuscate.file : CPP : OBFUSCATED_CPP ;

and import that module, you'll be able to use the rule "obfuscated-cpp" in Jamfiles, which will convert source to the OBFUSCATED_CPP type.
The remaining method is to declare your own main target class. The simplest example of this can be found in "build/alias.jam" file. The current V2 uses this method when transformations are relatively complex. However, we might deprecate this approach. If you find that you need to use it (that is, the first two approaches are not sufficient), please let us know by posting to the mailing list.

Toolset modules
If your extensions will be used only on one project, they can be placed in a separate .jam file which will be imported by your project-root.jam. If the extensions will be used on many projects, the users will thank you for a finishing touch.
The standard way to use a tool in Boost.Build is the using rule. To make it work, you module should provide an init rule. The rule will be called with the same parameters which were passed to the using rule. The set of allowed parameters is determined by you. For example, you can allow the user to specify paths, tool version, or tool options.
Here are some guidelines which help to make Boost.Build more consistent:
The init rule should never fail. Even if user provided a wrong path, you should emit a warning and go on. Configuration may be shared between different machines, and wrong values on one machine can be OK on another.
Prefer specifying command to be executed to specifying path. First of all, this gives more control: it's possible to specify

/usr/bin/g++-snapshot
time g++

as the command. Second, while some tools have a logical "installation root", it better if user don't have to remember if a specific tool requires a full command or a path.
Check for multiple initialization. A user can try to initialize the module several times. You need to check for this and decide what to do. Typically, unless you support several versions of a tool, duplicate initialization is a user error. If tool version can be specified during initialization, make sure the version is either always specified, or never specified (in which case the tool is initialied only once). For example, if you allow:

using yfc ;
using yfc : 3.3 ;
using yfc : 3.4 ;

Then it's not clear if the first initialization corresponds to version 3.3 of the tool, version 3.4 of the tool, or some other version. This can lead to building twice with the same version.
If possible, the init must be callable with no parameters. In which case, it should try to autodetect all the necessary information, for example, by looking for a tool in PATH or in common installation locations. Often this is possible and allows the user to simply write:

using yfc ;

Consider using facilities in the tools/common module. You can take a look at how tools/gcc.jam uses that module in the init rule.